A team of international scientists have discovered the first Earth-sized planet in our galaxy that has a high chance of harbouring life.

The exoplanet – the name given to planets outside our solar system – was spotted orbiting the "habitable" zone of a star some 500 light years from Earth

Astronomers have named the planet Kepler-186f after the Kepler telescope that was used to find it.

The planet's position from its star is similar to Earth's distance from the Sun, meaning Kepler-186f could potentially hold liquid water, the crucial condition for the development of life.

"We can now say that other, potentially habitable worlds, similar in size to Earth, can exist… It's no longer in the realm of science fiction," said Nasa's Elisa Quintana.

An artists impression of 'Earth-like' planet Kepler-186f

"The planet is the right size and is at the right distance to have properties that are similar to our home planet," added Ms Quintana, the lead author of a paper setting out the discovery which was published in the journal Science.

Of nearly 1,800 planets detected over the past 20 years, about 20 orbit within the "habitable zone" but many are at least one-and-a-half times larger than Earth, which often attracts a thick layer of hydrogen and helium.

Because Kepler-186f is only slightly larger than Earth, scientists can better predict the composition of its surface and atmosphere.

Kepler-186f is the fifth and outermost planet orbiting the Kepler-186 star, right on the far edge of that solar system's habitable zone, meaning the surface temperature might not be warm enough to stop water from freezing.

The planet was discovered when astronomers noticed the shadow it cast on its star. Current technology does not allow astronomers to see the celestial body itself or conduct analysis to determine beyond doubt its atmosphere or composition.

"Some people call these habitable planets, which of course we have no idea if they are," said Stephen Kane, another member of the team.

"We simply know that they are in the habitable zone, and that is the best place to start looking for habitable planets," he added.

"However, it is also slightly larger than the Earth, and so the hope would be that this would result in a thicker atmosphere that would provide extra insulation."

The Kepler space telescope launched in March 2009 and is used to observe some 150,000 stars, a few thousand of which have been found to have possible planets.